The present invention relates in general manner to disuniting thin layers in the context of fabricating semiconductor substrates for the microelectronic, optoelectronic, and optic components and related uses.
The present invention also relates to a novel tool for separating a structure in controlled and measured manner at the plane of an interface, in particular at an interface that has been weakened by implanting species using a known method such as the SMART-CUT® method disclosed for example in U.S. Pat. No. 5,374,564.
It is initially recalled that that method implements fracturing at an interface or weakened zone that is obtained by implanting species, typically ions of hydrogen and/or rare gases into a semiconductor material structure or wafer that has been attached by molecular bonding to another wafer that acts as a support. Thus, the SMART-CUT® method makes it possible to provide thin films and to assemble them together in stacked structures from which they can be removed.
The steps of that method are summarized below. Initially, an intermediate bonding layer (typically silica, silicon nitride, palladium, etc.) is generally formed by deposition (or by thermal oxidation for silica on) at least one of the wafers that are to be assembled together. Ions are then implanted using a beam of ions at a single energy over the entire surface of the wafer that is to be made thinner. The implanted ions weaken the material at an interface whose depth is a function of the implantation energy and is typically of the order of 1 micrometer (μm).
The surfaces of the wafers to be assembled together are subjected to treatment (mechanical and chemical lapping, chemical treatment, etc.) prior to bonding. The implanted plate is then attached to a stiffening substrate by molecular bonding. The bonding force can be increased by annealing for consolidation purposes. Thereafter, the implanted film is fractured at the weakened zone. This fracturing can be achieved in various ways, generally by applying thermal or mechanical energy or both. The final step consists in lapping the surface of the thinned-down film by polishing or lapping either chemically or both chemically and mechanically.
Thus, one of the essential steps of that method is fracturing in the plane of the weakened zone, which step generally relies on the principle of supplying thermal and/or mechanical energy. With thermal fracturing, the implanted species migrate in the plane of the implanted zone and form cavities of gas (a phenomenon referred to as “ripening”). Bonded to a stiffening substrate, the implanted film has its cavities grow preferentially in the plane of the interface where the density of the implanted species is greatest. The last stage of ripening corresponds to the cavities coalescing. Their diameter can then be as great as several micrometers. Forming these cavities serves to further weaken the interface. Forming gas inside the cavities generates pressure that encourages fracturing.
Fracturing by heat treatment is advantageous for industrial implementation because it requires minimal physical handling, however it cannot be achieved with all combinations of materials. Thus, heterostructures possessing layers of materials having very different thermal expansion coefficients (TECs) (e.g. silicon on quartz or sapphire on silicon) cannot be subjected to the heat treatment needed for fracturing without causing irremediable damage to the structure (warping or rupturing). By way of example, a structure comprising silicon on quartz having respective thermal expansion coefficients of 2.6×10−6 centimeters per degree kelvin (cm/K) and 0.5×10−6 cm/K breaks prior to reaching the threshold temperature. Fracturing must therefore be finished off mechanically.
In addition, the techniques used in fabricating substrates of the silicon on insulator (SOI) structures require ever finer control over bonding energies.
Bonding can rely on a variety of techniques: molecular bonding (directly or via transition layers); metal bonding; fusion bonding; etc. Bonding energy per unit area depends on numerous parameters: the selected material; the planeness or smoothness of wafer surfaces; roughness; bonding temperature; heat budget of the consolidation treatment; etc.
Studies in this field are therefore very precious in developing products. They make it possible to determine the influence of numerous parameters. However, at present, there are no tools or techniques available on an industrial scale that enable reliable and reproducible measurements of bonding energy to be obtained.
In this regard, and with reference to FIG. 1 of the drawing figures, the preferred prior art technique for commercial use includes inserting a blade 10 at the desired interface between two wafers 11 and 12, and then disbanding the structure in part over a distance that is measured, thus making it possible ultimately to determine the bonding energy.
Still in the context of disuniting wafers, it can be necessary during the technological steps of creating a component to remove a substrate used at the beginning of the method. By way of example, materials having a large forbidden band (based on gallium nitride or other metal nitrides) can be grown epitaxially in industry on a sapphire substrate. After epitaxy, the insulation quality of the substrate prevents any electrical contact being made with the rear face. Thus, when it is desired to use such epitaxy to make a component of vertical geometry (for example a light-emitting diode (LED), or a laser source having a vertical cavity), it can be useful or even essential to remove the substrate. Various technologies have been developed for this purpose: selective chemical etching; mechanical or ion thinning; and the so-called “laser lift-off” technique. This technique consists in disuniting a heteroepitaxial layer from its substrate by using a laser to scan the interface between the substrate and the epitaxially-grown layer.
However all the techniques that have been developed for removing a support that is no longer desired or needed present certain limitations. The technique of chemically etching the substrate destroys it, thus wasting material. Also, the laser life-off technique can generally be performed only over small areas, and not over the entire surface of a substrate having a diameter of about ten centimeters or more.
In order to mitigate these limitations, techniques have begun to be developed involving a “dismountable” substrate. In general, a dismountable substrate presents a multilayer structure: a substrate for epitaxial growth which is of small thickness (typically a few nanometers), providing a lattice parameter that is adapted to epitaxial growth is bonded to a mechanical support that is thick (typically a few hundreds of micrometers). After epitaxy, the idea is to dissociate the two layers of the resulting pseudosubstrate. That technology requires precise control over bonding energy as a function of temperature. More precisely, the bonding energy must be strong enough to accept the temperature required for epitaxial growth and weak enough subsequently to allow the layers to be disunited.
It is then possible to dissociate the pseudosubstrate by applying stress of a different kind, for example mechanical stress. The various techniques presently in existence for disuniting layers are summarized below.
Firstly, as mentioned above, thermal fracturing is typically used in the fabrication of SOI materials. It is obtained by high temperature annealing (typically at a temperature greater than 400° C.). That technique presents several advantages: it is easy to implement industrially, it is repeatable, and the surfaces after fracture are uniform. In addition, high speed annealing furnaces enable high rates of throughput to be achieved.
For mechanical fracturing, there presently exist various ways of proceeding with mechanical disjunction of thin layers. U.S. Pat. No. 6,468,879 describes a tool and a technique in which the structure is disbanded in the weakened interface plane by applying localized deformation action. It uses arms that hold onto opposite sides of the structure by suction, and a trigger system that initiates disbanding by moving the edges of the wafer apart. This localized effect then propagates as a disbanding front to produce disbanding over the entire interface.
A limitation of that approach lies in that it is suitable only for structures having low disbanding energy such as SOI structures made using the SMART-CUT® method, where the energy required for disbanding is greatly reduced by prior heat treatment. For higher disbanding energies, the deformation of the wafers becomes large and can go so far as to damage them. In addition, certain semiconductor materials such as InP present a lower plastic deformation threshold and cannot be used with that kind of technique.
In addition, because of the manual action of the disbanding force, that system does not provide any measurement of the previously-existing bonding energy.
Secondly, British patent application GB-A-2,124,547 discloses a method of cleaving plates that are laminated parallel to their surfaces and describes a tool having grippers that can exert separation stress by applying suction to the plates that are to be cleaved. The grippers can move in parallel or they can pivot about a common axis. Provision is made for the grippers to be ring-shaped.
Nevertheless, the technique described in that document has a metallurgical application and does not appear to be suitable for fragile materials such as semiconductors. In addition, the roughness of the surfaces obtained after such cleaving made in accordance with the teaching of that document appears to be incompatible with present specifications that apply in the field of semiconductors (using magnitudes of angstrom order). Finally, that document does not make provision for measuring bonding energy, nor for measuring the imposed spacing.
Thirdly, a cutting technique using a jet of liquid under pressure is used in a so-called “Eltran” method which consists in growing a film of silicon epitaxially, in bonding it to a mechanical support of silicon oxide, and then in separating the epitaxial film by cutting using a jet of water which can be assisted by other techniques such as inserting blades.
Fourthly, and returning to FIG. 1, when it comes to measuring bonding energy (or surface energy), the technique in which two bonded-together wafers 11 and 12 of respective thicknesses tw1 and tw2 and with respective Young's moduli E1 and E2 are separated in part by means of a separator such as a razor blade 10 makes it possible by measuring the disbanding length L to calculate the bonding energy using a mathematical formula. In practice, a blade is selected that is of thickness which depends on the stiffness and the size of the bonded substrates. The blade is inserted into the junction, thereby causing partial disbanding. Once this disbanding has stabilized, the disbanded length is measured. The equations enable the bonding energy to be calculated.
However, whether for disbanding implemented in a method of fabrication or used for measuring bonding energy, recourse to blades presents certain limits. Firstly, it is always desirable to initiate disbanding in the weakest region of the implanted interface, and it is difficult to position the blade precisely so as to initiate fracturing at exactly this position. The use of a blade also incurs the risk of scratching the facing surfaces while they are being separated. In addition, when the radius of curvature of the wafers that are being separated becomes too great, such deformation of the wafers can give rise to structural defects such as dislocations. Finally, the principle of that technique does not enable the disbanding energy implemented to be measured in a manner that is sufficiently precise and reproducible, even though attempts have been made in the prior art described above to achieve this with the help of a mathematical model.
Throughout the specification below, the term “disuniting” is used generally. This term thus covers the notion of disbanding structures that have been assembled together (bonding by means of adhesive, of molecular bonding, optionally assisted by surface treatment such as plasmas, of metal bonding, of fusion bonding, etc.). However the term “disuniting” is also used to designate fracturing of the type involving cleaving in a plane parallel to the interface, with a particular example being given by SMART-CUT® method fracturing at the weakened interface, or indeed lift-off at the interface between a substrate and a layer that has been deposited, epitaxially or otherwise.